Rollup merge of #96704 - GuillaumeGomez:rotation-animation, r=jsha

Add rotation animation on settings button when loading

As discussed, I added an animation when the settings JS file is loading (I voluntarily made the timeout at the end of the `settings.js` super long so we can see what the animation looks like):

https://user-images.githubusercontent.com/3050060/166693243-816a08b7-5e39-4142-acd3-686ad9950d8e.mp4

r? ````@jsha````

src/librustdoc/html/static/js/main.js

@@ -301,6 +301,7 @@ function loadCss(cssFileName) {
     }
 
     getSettingsButton().onclick = event => {
+        addClass(getSettingsButton(), "rotate");
         event.preventDefault();
         loadScript(window.settingsJS);
     };